Two New Calcium-rich Gap Transients in Group and Cluster Environments

February 2017 • 2017ApJ...836...60L

Authors • Lunnan, R. • Kasliwal, M. M. • Cao, Y. • Hangard, L. • Yaron, O. • Parrent, J. T. • McCully, C. • Gal-Yam, A. • Mulchaey, J. S. • Ben-Ami, S. • Filippenko, A. V. • Fremling, C. • Fruchter, A. S. • Howell, D. A. • Koda, J. • Kupfer, T. • Kulkarni, S. R. • Laher, R. • Masci, F. • Nugent, P. E. • Ofek, E. O. • Yagi, M. • Yan, Lin

Abstract • We present the Palomar Transient Factory discoveries and the photometric and spectroscopic observations of PTF11kmb and PTF12bho. We show that both transients have properties consistent with the class of calcium-rich gap transients, specifically lower peak luminosities and rapid evolution compared to ordinary supernovae, and a nebular spectrum dominated by [Ca II] emission. A striking feature of both transients is their host environments: PTF12bho is an intracluster transient in the Coma Cluster, while PTF11kmb is located in a loose galaxy group, at a physical offset ∼150 kpc from the most likely host galaxy. Deep Subaru imaging of PTF12bho rules out an underlying host system to a limit of {M}_R> -8.0 {mag}, while Hubble Space Telescope imaging of PTF11kmb reveals a marginal counterpart that, if real, could be either a background galaxy or a globular cluster. We show that the offset distribution of Ca-rich gap transients is significantly more extreme than that seen for SNe Ia or even short-hard gamma-ray bursts (sGRBs). Thus, if the offsets are caused by a kick, they require higher kick velocities and/or longer merger times than sGRBs. We also show that almost all Ca-rich transients found to date are in group and cluster environments with elliptical host galaxies, indicating a very old progenitor population; the remote locations could partially be explained by these environments having the largest fraction of stars in the intragroup/intracluster light following galaxy-galaxy interactions.

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Frank Masci

Senior Scientist